1. Field of the Invention
The present invention relates to a fuel cell stack provided with a power-generating cell having a joined unit including an electrolyte interposed between an anode electrode and a cathode electrode, the joined unit being interposed between separators so that fuel gas is supplied to the anode electrode, while oxygen-containing gas is supplied to the cathode electrode.
2. Description of the Related Art
For example, the phosphoric acid fuel cell (PAFC) is provided with a power-generating cell constructed such that a joined unit, which comprises an anode electrode and a cathode electrode principally composed of carbon respectively and provided opposingly on both sides of an electrolyte layer composed of a silicon carbide porous material (matrix) impregnated with concentrated phosphoric acid, is interposed between separators (bipolar plates). Usually, a predetermined number of the power-generating cells are stacked to be used as a fuel cell stack.
On the other hand, the solid polymer fuel cell (SPFC) adopts an ion exchange membrane composed of a solid polymer ion exchange membrane (cation ion exchange membrane). Similarly, the solid polymer fuel cell is constructed by stacking a predetermined number of power-generating cells each comprising separators and a joined unit composed of the ion exchange membrane as described above, and it is used as a fuel cell stack.
In such a fuel cell stack, a fuel gas such as a gas principally containing hydrogen (hereinafter referred to as xe2x80x9chydrogen-containing gasxe2x80x9d as well), which is supplied to the anode electrode, contains hydrogen which is ionized into ion on the catalyst electrode, and the ion is moved toward the cathode electrode via the electrolyte. The electron, which is generated during this process, is extracted for an external circuit, and the electron is utilized as DC electric energy. An oxygen-containing gas such as a gas principally containing oxygen or air (hereinafter referred to as xe2x80x9coxygen-containing gasxe2x80x9d as well) is supplied to the cathode electrode. Therefore, the hydrogen ion, the electron, and the oxygen are reacted with each other on the cathode electrode, and thus water is produced.
As for the fuel cell as described above, in order to exhibit the effective power generation performance, the optimum operation temperature is set. In order to maintain the power-generating cell at the operation temperature, a variety of cooling structures are adopted. In general, the following structure is known. That is, a cooling medium passage is formed on the separator for constructing the fuel cell stack. A cooling medium such as water is supplied to the passage, and thus the power-generating cell is cooled.
In this case, the water to be used as the cooling medium as well as a general coolant to be used for a cooling structure for an automobile is contaminated with any impurity such as ion and any metal-based additive. The conductivity is given to the coolant or the cooling medium itself. On the other hand, also when deionized water or pure water is used for the coolant, then any metal or the like is mixed therein during the circulation through a cooling piping and through a radiator during the operation, and the conductivity is consequently given to the coolant.
However, in the case of the fuel cell stack, the electrons, which are generated in the respective power-generating cells, are extracted from collecting electrodes which are disposed on both end sides. Therefore, if the conductivity is given to the cooling medium as described above, the electricity flows through the cooling medium. Therefore, the following problem is pointed out. That is, the electricity flows, for example, through the cooling piping and through the radiator by the aid of the cooling medium. As a result, the earth fault or the liquid junction takes place, resulting in the decrease in output of the entire fuel cell stack.
A principal object of the present invention is to provide a fuel cell stack which makes it possible to reliably avoid any electric leakage via a cooling medium and which makes it possible to maintain effective power generation performance with a simple structure.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.